Ehrlichia canis is a tick-transmitted obligately intracellular bacterium that causes moderate-to-severe and sometimes fatal disease in wild and domestic canids. The genomes of E. canis and other organisms in the genus, including E. chaffeensis and E. ruminantium, exhibit a high degree of genomic synteny, paralogous protein families, a large proportion of proteins with transmembrane helices and/or signal sequences, and a unique serine-threonine bias associated with potential for O-glycosylation and phosphorylation, and have tandem repeats and ankyrin domains in proteins associated with host-pathogen interactions (Collins et al., 2005; Hotopp et al., 2006; Frutos et al., 2006; Mavromatis et al., 2006). A small subset of the more than 900 proteins encoded by each of these genomes are recognized by antibody (Doyle et al., 2006; McBride et al., 2003; McBride et al., 2000; Sumner et al., 2000). Several of the major immunoreactive proteins identified and molecularly characterized are serine-rich glycoproteins that are secreted. Many of these glycoproteins have tandem repeats; however, one has numerous eukaryote-like ankyrin domains (Doyle et al., 2006; McBride et al., 2003; McBride et al., 2000; Nethery et al., 2005; Singu et al., 2005; Yu et al., 2000).
Numerous proteins have been identified in E. canis (n=12) and E. ruminantium (n=31) that contain tandem repeats. Notably, three immunoreactive proteins with tandem repeats have been identified and molecularly characterized in E. chaffeensis (gp120, gp47, and VLPT) as well as two orthologs in E. canis (gp140 and gp36, respectively). The ortholog of E. chaffeensis vlpt gene has not been identified in E. canis, and it has been reported that this gene is not present in other ehrlichial genomes (Hotopp et al., 2006). Extensive variability in the number and/or sequence of tandem repeats in the E. chaffeensis immunoreactive proteins (gp120, gp47 and VLPT) as well as E. canis gp36 is well documented (Chen et al., 1997; Doyle et al., 2006; Sumner et al., 1999). The presence of tandem repeats in both coding and noncoding regions of the genome has been linked to an active process of expansion and reduction of ehrlichial genomes (Frutos et al., 2006) and is considered a major source of genomic change and instability (Bzymek and Lovett, 2001).
Although the E. chaffeensis VLPT is immunoreactive, little is known regarding its cellular location, function and role in development of protective immunity. The E. chaffeensis vlpt gene exhibits variations in the number of 90-bp tandem repeats (3 to 5) and has been utilized as a molecular diagnostic target and for differentiation of isolates (Sumner et al., 1999; Yabsley et al., 2003). The VLPT of E. chaffeensis Arkansas is a 198 amino acid protein that has four repeats (30 amino acids) and has a molecular mass approximately double that predicted by its amino acid sequence (Sumner et al., 1999). E. chaffeensis VLPT protein appears to have posttranslational modification consistent with other described ehrlichial glycoproteins, but the presence of carbohydrate on VLPT has not been demonstrated.
The present invention fulfills a need in the art by providing novel methods and compositions concerning erhlichial infections in mammals, and in particular provides methods and compositions in an E. canis ortholog of E. chaffeensis VLPT.